1. Field of the Invention
The present invention relates to a semiconductor device.
In this specification, a “semiconductor device” refers to any device which can function by utilizing semiconductor characteristics, and a memory device, an electro-optical device, a semiconductor circuit, an electronic component, and an electronic device are each an embodiment of the semiconductor device.
2. Description of the Related Art
A memory device is one of semiconductor devices including semiconductor elements. Memory devices are broadly classified into two categories: a volatile memory device that loses stored data when supply of power is stopped, and a non-volatile memory device that holds stored data even after supply of power is stopped.
Typical examples of volatile memory devices are dynamic random access memories (DRAMs) and static random access memories (SRAMs). Such volatile memory devices lose stored data when supply of power is stopped but consume relatively less power because they do not need high voltage as in non-volatile memories.
The area of a DRAM can be small because one memory element of the DRAM includes only one transistor and one capacitor; however, a data retention period is extremely short, frequent refresh operation is required, and power consumption cannot be sufficiently reduced.
Although an SRAM can operate at high speed, the area of the SRAM is large because one memory element of the SRAM includes at least six transistors. Further, since the off-state current of the transistors are increased due to miniaturization of the transistors, power consumption during a data retention period cannot be sufficiently reduced.
A typical example of a non-volatile memory device is a flash memory. A flash memory holds electric charge in a floating gate and thus has a semi-permanent data retention period (e.g., see Patent Document 1). However, a flash memory consumes large power because it requires high voltage for writing and erasing data, and in addition, it is not easy to increase the speed of these operations. Further, in writing and erasing data, electric charge is injected to a floating gate by generating tunneling current by application of a high electric field to an insulating film, so that degradation of the insulating film proceeds in accordance with the number of writing cycles.
It has recently been found that a transistor formed using an oxide semiconductor with a wide band gap has significantly high off-state resistance, and it has been proposed that the transistor is used to form a memory element or a signal processing circuit which is used for a memory device (see Patent Documents 2 to 4).
Owing to the high off-state resistance of the transistor in such a memory element, it takes a long time for electric charge accumulated in a capacitor connected to the transistor in series to be lost, and this makes it possible to reduce consumption of current required for a general flip-flop circuit in an SRAM or the like to hold data and to further reduce power consumption. Alternatively, a very large capacitor required for a DRAM is not necessary, Which allows the size of a circuit to be reduced, the manufacturing process to be simplified, and the yield to be improved.